Broadband polarizer made using ion exchangable fusion drawn glass sheets

ABSTRACT

The disclosure is directed to broadband, glass optical polarizers and to methods for making the glass optical polarizers. The glass optical polarizer includes a substantially bubble free fusion drawn glass having two pristine glass surfaces and a plurality of elongated zero valent metallic particle polarizing layers.

This application is a divisional of and claims the benefit of priorityunder 35 U.S.C. § 120 of U.S. application Ser. No. 14/483,306 filed onSep. 11, 2014, which claims the benefit of priority of U.S. ApplicationSer. No. 61/878,720 filed on Sep. 17, 2013 the content of which isrelied upon and incorporated herein by reference in its entirety.

FIELD

This disclosure is directed to glass polarizers made fromion-exchangeable fusion drawn sheet glass, and a method of making suchpolarizers using ion-exchangeable fusion drawn glass sheets. Thepolarizers described herein are broadband glass polarizers operative inthe infrared region of the electromagnetic spectrum.

BACKGROUND

Glass optical polarizers are made by a number of different corporationsusing different methods. Among these glass optical polarizers are thePolarcor™ (Corning Incorporated, Corning, N.Y.) glass polarizingproducts which are formed by heat treating a glass containing silver andhalide ions to precipitate silver halide particles, which particles arethen stretched in a redraw step which is followed by a reduction step inwhich the silver halide particles are reduced to form anisotropic silverparticles in the glass. Other polarizers are made from sodium silicate(soda lime) glass by ion exchanging silver ions for sodium ions near theglass surface, followed by heat treating and drawing the soda lime glassto elongate silver particles in the glass.

While the presently available polarizers have served industry well, withincreasing miniaturization, particularly in the telecommunicationsindustry, there is a need for glass optical polarizers that are boththin and strong. The present disclosure is directed to such polarizerproducts and to a method for making the same.

SUMMARY

According to an embodiment of the present disclosure, a glass opticalpolarizer is provided. The glass optical polarizer includes asubstantially bubble free fusion drawn glass having two pristine glasssurfaces and a plurality of elongated zero valent metallic particlepolarizing layers.

According to another embodiment of the present disclosure, a method formaking a glass optical polarizer is provided. The method includesproviding a fusion drawn alkali-containing glass substrate andion-exchanging a noble metal ion into the glass substrate using anion-exchange molten metal salt bath. The molten metal salt bath includesa noble metal salt and an alkali metal salt. The method further includesremoving excess ion-exchange salts from the glass substrate, andreducing the noble metal ion in hydrogen to form a plurality of zerovalent metallic particle polarizing layers. The method further includesredrawing the glass substrate to form a plurality of elongated zerovalent metallic particle polarizing layers. The glass optical polarizerhas two pristine glass surfaces and is substantially bubble free.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating contrast ratio (in dB) versus wavelengthof redrawn glass polarizers prepared as described herein.

FIG. 2 is a graph illustrating % transmission versus wavelength ofredrawn glass polarizers prepared as described herein.

FIG. 3 is an SEM (scanning electron microscopy) photograph of the crosssection of polarizing glass prepared as described herein after thehydrogen reduction.

FIG. 4A is an SEM photograph of polarizing glass prepared as describedherein after hydrogen reduction at 420° C. for 20 hours and stretchingat a tension of 6000 psi.

FIG. 4B is an SEM photograph of polarizing glass prepared as describedherein after hydrogen reduction at 420° C. for 20 hours and stretchingat a tension of 2000 psi.

FIG. 4C is an SEM photograph of polarizing glass prepared as describedherein after hydrogen reduction at 420° C. for 20 hours and stretchingat a tension of 4000 psi.

FIG. 5 is a graph illustrating sodium and potassium ion concentrationfor two different glass samples.

FIG. 6A is the maximum transmission for the glass polarizer of FIGS. 4A,4B and 4C.

FIG. 6B is the minimum transmission for the glass polarizer of FIGS. 4A,4B and 4C.

FIG. 7 depicts a prior art polarizer.

DETAILED DESCRIPTION

Herein, contrast of a polarizer is the ratio T_(max)/T_(min) and thecontrast ratio is the log₁₀ (T_(max)/T_(min)), where T_(max) is themaximum transmission value and T_(min) is the minimum transmissionvalue.

This disclosure is directed to glass optical polarizers made from fusiondrawn glass, and to a method for making the polarizers usingion-exchange. This disclosure describes a process by which metal ionsare ion-exchanged into a fusion drawn glass sheet, and then theion-exchanged glass sheet is further processed to make glass opticalpolarizers. Metal ions such as, but not limited to, noble metal ions canbe ion-exchanged into the glass to create optical polarizers. For thepurposes of the present disclosure, noble metals include, but are notlimited to, copper, gold, iridium, mercury, osmium, palladium, platinum,rhenium, rhodium, ruthenium, and silver.

The polarizers disclosed herein may have a thickness of about 100 μm toabout 350 μm and a contrast ratio of at least about 20 dB over awavelength range of about 900 nm to about 2,000 nm. The polarizers mayhave a thickness of about 200 μm to about 300 μm. Alternatively, thepolarizers may have a contrast ratio of at least about 30 dB over awavelength range of about 1,000 nm to about 2,000 nm. The polarizers canbe made of any fusion drawn glass containing ion-exchangeable alkalimetal ions, for example but without limitation, an alkali-containingaluminosilicate glass, alkali-containing aluminoborosilicate glass andsoda lime glass. The glass may be fusion drawn glass containingexchangeable alkali metal ions. Exemplary glass compositions can befound in U.S. Pat. Nos. 8,431,301 and 8,158,543, and in U.S. PatentApplication Publication Nos. 2011/0045962, 2011/0201490, 2013/0122284and 2013/0122313, the specifications of which are incorporated byreference in their entirety.

The polarizers disclosed herein may include a plurality of elongatedzero valent metallic particle polarizing layers having a thickness ofabout 0.2 μm to about 0.5 μm. The metallic particle polarizing layersmay be situated within about 1.0 μm to about 2.0 μm of at least one ofthe pristine surfaces of the fusion drawn glass. Some of the metallicparticle polarizing layers may be situated within about 1.0 μm to about2.0 μm of a first of the two pristine surfaces, and some of the metallicparticle polarizing layers may be situated within about 1.0 μm to about2.0 μm of a second of the pristine surfaces. The metallic particlescorrespond to the metal ions which are ion-exchanged into the glass.

The ion-exchanged, optical polarizers disclosed herein may find utilityin the infrared (IR) region and can be used in, among other things,telecommunications and supporting equipment. The polarizers describedherein may be thin, broadband final products that have a good qualitysurface finish. Due to the fact they are made from fusion drawn glass,the polarizers do not require additional finishing steps. The use offusion drawn glass imparts characteristics to the surface of thefinished glass, as well as to the internal glass network structure, thatcannot be obtained using other types of glass, for example, slot drawnglass, float glass and cast glass.

A method for making a glass optical polarizer is provided herein. Themethod described herein does not affect the pristine nature of thefusion drawn glass surfaces and does not introduce scratches,sub-surface defects, or bubbles into the formed glass. The method mayinclude providing a fusion drawn alkali-containing glass sheet as asubstrate. The sheet may have a thickness of about 0.5 mm to about 2.0mm. The composition of the provided glass sheet may be, but is notlimited to, alkali-containing aluminosilicate and alkali-containingaluminoborosilicate glasses.

The method may further include ion-exchanging a noble metal ion into thefusion glass substrate using an ion-exchange molten metal salt bath. Themolten metal salt bath may include a noble metal salt. The molten metalsalt bath may optionally include an alkali metal salt in which the noblemetal salt may be dissolved. At least one of the noble metal salt andthe alkali metal salt may be, for example, a nitrate salt. As a furtherexample, the noble metal salt may be a silver nitrate salt and thealkali metal salt may be a potassium nitrate salt.

Examples of ion-exchange methods are described in U.S. Pat. No.8,312,739, and U.S. Patent Application Publication Nos. U.S.2008/00407300 and U.S. 2009/0142568, the specifications of which areincorporated by reference in their entirety.

Ion-exchanging a noble metal ion into the fusion glass substrate may bedone, for example, for a period of about 1.0 hour to about 20 hours andat a temperature of about 300° C. to about 500° C. The time period andthe temperature during ion-exchange may be varied in order to modify thedepth to which the noble metal ion diffuses into the fusion glasssubstrate.

The method may further include removing excess ion-exchange salts fromthe ion-exchanged substrate to thereby yield a fusion glass substratehaving a noble metal ion therein.

The method may further include reducing the noble metal ion in purehydrogen to form a fusion drawn glass substrate having a plurality ofzero valent metallic particle polarizing layers. Reducing the noblemetal ion may be done at a pressure of about 1.0 atm to about 20 atm.The pressure may be varied as desired, as higher pressures may be usedto affect the rate of reduction. Reducing the noble metal ion may bedone at a temperature of about 350° C. to about 550° C. Further,reducing the noble metal ion may be done for a time of about 1.0 hour toabout 30 hours.

The method may further include redrawing the fusion drawn glasssubstrate having a plurality of zero valent metallic particle polarizinglayers. Redrawing the fusion glass substrate may be done by heating andstretching the substrate at a viscosity of 10⁵-10¹⁰ poise (e.g., 10⁶-10⁷poise or 10⁷-10⁹ poise). Redrawing the fusion glass substrate may bedone, for example, at a temperature above the annealing point Tameal(i.e., a glass viscosity less than 10⁻¹³ poise), but at least about 50°C. below the softening point T_(soft) of the glass (i. e., glassviscosity above 10^(−7.5) poise). The annealing point T_(anneal) may beabout 550° C. to about 650° C. (for example 590° C.<T_(anneal)<610° C.);and the softening point T_(soft) may be about 800° C. to about 900° C.(for example 860° C.<T_(soft)<870° C.).

Redrawing the fusion drawn glass substrate may reduce the scale(primarily the thickness) of the substrate to form a glass opticalpolarizer having a thickness of about 300 μm or less. Redrawing thefusion drawn glass substrate may also yield a stretched microstructureof the noble metal particles embedded in the substrate, which in turnyields polarizing properties suitable for glass optical polarizers. Theresultant glass optical polarizer may have two pristine glass surfacesand may be substantially bubble free.

Examples of a redraw process are shown in U.S. Pat. Nos. 7,510,989 and7,618,908, the specifications of which are incorporated by reference intheir entirety.

After redrawing the fusion drawn glass substrate, the resultingpolarizers may have a thickness of about 400 μm or less. Alternatively,the thickness may be about 300 μm or less. The polarizers describedherein have a transmission of at least about 80% over a wavelength rangeof about 1,000 nm to about 2,100 nm. The polarizers may have an averageredrawn thickness of about 300 μm or less with a good quality surfacefinish that does not require additional finishing steps due to the factthat the polarizers are formed from fusion drawn glass.

Subsequent to redrawing the fusion drawn glass substrate, the methoddisclosed herein may further include chemically strengthening the glassoptical polarizer. Chemically strengthening may include ion-exchanging alarger alkali metal ion for a smaller alkali metal ion in the fusionglass substrate using an ion-exchange molten metal salt bath. The alkalimetal ion may be, for example, a potassium ion, and the smaller alkalimetal ion may be, for example, a sodium ion. The molten metal salt bathmay include an alkali metal salt. The alkali metal salt may be, forexample, a nitrate salt. As a further example, the alkali metal salt maybe a potassium nitrate salt. Chemically strengthening may restore anycompressive strength that is lost during hydrogen reduction or duringthe redrawing of the fusion drawn glass substrate and provide a strong,damage resistant glass optical polarizer.

As previously mentioned, fusion drawn glass has two pristine glasssurfaces and the method used to make the polarizers described hereindoes not substantially affect the pristine surfaces. The fusion drawprocess has been described in U.S. Pat. Nos. 3,338,696, 3,682,609, and6,974,786, and in U.S. Patent Application Publication No. 2008/0047300,the specifications of which are incorporated by reference in theirentirety, and has been employed in the production of active-matrixliquid-crystal-displays (AMLCD). Although display applications dictatethe use of alkali-free glass, optical polarizers are not limited toalkali-free glass.

The fusion draw process imparts characteristics to fusion drawn glassnot observed in other glass. For example, the process provides precisegeometry control which enables uniform glass thickness. The process alsoeliminates bubbles (also called seeds, voids or inclusions) within theglass. Bubbles can affect light transmission through glass by changingthe angle of the light's path as is passes from glass to void and backinto glass. This causes birefringence in the glass which affects lighttransmission by introducing scattering and detrimentally affects opticalpolarizers, particularly when used in telecommunications equipment. Thepristine surfaces of the fusion drawn glass are substantially flat,smooth, and substantially free of any surface contamination in theas-formed state. Furthermore, fusion drawn glass does not containsurface roughness or sub-surface flaws such as arise in any grindingand/or polishing procedure. Unlike other glass processes the fusion drawprocess buries the forming surfaces of the glass that contact theforming surface of an element, called an isopipe, within the formedsheet of glass. Thus the exterior surfaces of the glass are contact freeand pristine.

The use of thin (about 0.5 mm to about 2.0 mm) fusion drawn glass as thestarting glass material provides optical polarizers having high qualitypristine surfaces (without the need for extensive post processing suchas grinding and polishing) and having final thicknesses of about 300 μmor less, a contrast ratio of at least about 20 dB and a transmission ofat least about 75% over a wavelength range of about 900 nm to about2,000 nm. Optical polarizers as described herein may also have atransmission of greater than about 82% over a wavelength range of about1,000 nm to about 2,000 nm. Within the telecommunications range of about1,300 nm to about 1,550 nm, the transmission may be greater than about85%. The contrast ratio may be equal to or greater than about 25 dB overa wavelength range of about 900 nm to about 2,000 nm. Alternatively, thecontrast ratio may be greater than about 30 dB over a wavelength rangeof about 900 nm to about 2,000 nm.

Properties, for example, contrast ratio, of the polarizers describedherein, are less sensitive to pulling tensions such as those appliedduring the redraw process in which glass viscosity is 10⁵-10¹⁰ poise(for example 10⁵-10⁷, or 10⁷-10⁹). The viscosity may vary based on thecomposition of the glass stretched.

Examples

Embodiments of the present disclosure are further described below withrespect to certain exemplary and specific embodiments thereof, which areillustrative only and not intended to be limiting.

In the examples described herein, silver ions were used as the exemplarynoble metal ion-exchanged into the fusion drawn glass. However, othernoble metal ions can also be ion-exchanged into the fusion drawn glass.In addition, while an alkali-containing aluminoborosilicate glass wasused in the examples, other glasses, such as, but not limited to,alkali-containing aluminosilicate glass, can also be used. Exemplaryalkali-containing aluminoborosilicate and aluminosilicate glasses mayinclude, but are not limited to, those shown in Table I. Further, whilethe ion-exchange of noble metal ions and alkali metal ions into theglass is exemplified herein using a molten salt bath method, the ionscan also be exchanged into the glass using a paste that is applied tothe surface of the glass and then heated.

Table I lists a number of exemplary glass compositions that can be usedto make fusion drawn glass for use in an optical polarizer as describedherein. The general glass composition, before any ion-exchange,comprises about 60 mol % to about 70 mol % SiO₂, about 13 mol % to about15 mol % Na₂O, about 7.0 mol % to about 15 mol % Al₂O₃, less than about8.0 mol % B₂O₃, less than about 8.0 mol % MgO, less than about 2.0 mol %K₂O, less than about 1.0 mol % CaO and less than about 0.5 mol % SnO₂.

TABLE I Exemplary Compositions (in mol %) Used in the Examples Glass 1Glass 2 Glass 3 Glass 4 SiO₂ 64.44 64.65 66.00 69.17 Al₂O₃ 13.9 13.9310.26 8.53 B₂O₃ 7.15 5.11 0.58 0 Na₂O 14.03 13.75 14.23 13.94 K₂O 0.54 02.37 1.17 MgO 0.01 2.38 5.75 6.45 CaO 0.06 0.14 0.59 0.54 SnO₂ 0.12 0.080.21 0.19

The fusion drawn glass sheets used in the example were obtained using afusion draw process and were then ion-exchanged for a period of about 20hours at a temperature of about 420° C. in an ion-exchange bathcontaining about 2.0 wt. % AgNO₃ and about 98 wt. % KNO₃. Aftercompletion of the ion-exchange, the silver ions were reduced in about1.0 atm of pure hydrogen at a temperature of about 400° C. for about 10hours in a hydrogen furnace. The silver-containing glass sheets werethen stretched using a redraw process at various temperatures T rangingfrom about 750° C. to about 900° C. to yield various strips withdiffering amounts of stretching force applied to them. The contrastratio and % transmission measurement data for the resulting glasspolarizers are shown in FIGS. 1 and 2, respectively.

FIG. 3 is an SEM photograph of a hydrogen reduced silver-containingfusion drawn glass prepared according to the present disclosure. FIG. 3illustrates four bands or layers comprising zero valent metallicparticle layers. More specifically, FIG. 3 shows broken segments ofsilver-containing sheets of various sizes and shapes which appear asbright lines or broken line segments. When the glass of FIG. 3 isredrawn, the zero valent metallic particles become elongated, therebypolarizing the bands or layers. FIGS. 4A-4C show a variety of sizes andaspect ratios that lead to the broadband properties of the polarizerdisclosed herein. FIGS. 4A-4C exemplify the glass polarizers producedaccording to this disclosure. In contrast to what is illustrated inFIGS. 4A-4C, the prior art polarizers, as shown in FIG. 7, do not showthe broad range of particle size and shape.

The scientific literature indicates that the wavelength of maximumabsorption for an elongated noble metal nano-particle depends on itsaspect ratio. As used herein, the aspect ratio is the ratio oflength:diameter. As such, a distribution of aspect ratios of respectivenano-particles may provide high contrast and good transmission over awide wavelength range. Further, as shown in FIG. 3, a distribution ofsilver metal particle sizes in the hydrogen treated glass leads to acorresponding distribution in aspect ratios. It is believed that duringthe redraw process, the softened glass acts as a flowing fluid exertingelongation forces on the noble metal particle. Nano-sized particles canbe elongated in such a fluid flow field if the forces exerted by theflowing fluid are sufficient to overcome the restoring forces of surfacetension, which tend to maintain the particle's spherical shape. Theserestoring forces are proportional to the surface tension and inverselyproportional to the particle radius. Hence, the larger the particle, theless the restoring force and the more easily the particle can beelongated. As such, for a given stress during the redraw process, largerparticles will be elongated to higher aspect ratios. This is verified byFIGS. 4A-4C, which show the microstructure of glass polarizers after theredraw process at three different pull tensions.

FIG. 6A illustrates the maximum transmission for the glass polarizer ofFIGS. 4A, 4B and 4C. FIG. 6B illustrates the minimum transmission forthe glass polarizer of FIGS. 4A, 4B and 4C. As FIGS. 6A and 6B exhibitby their maximum and minimum transmissions, the three polarizers ofFIGS. 4A-C are deemed substantially equivalent.

It is believed that the production of the microstructure exhibited inFIG. 3 is the result of instability in a typical reaction-diffusionsystem, also known as Liesegang effect. The bands of silver particlesand line segments shown in FIG. 3 are also referred to as “Liesegangbands”. Without being held to any particular theory, it is believed thatthe explanation for this behavior or effect is that the fusion drawnglass compositions undergo a reaction-diffusion based instability duringhydrogen reduction leading to the production of layers of silverparticles that have a wide distribution of both size and aspect ratio.The appearance of the Liesegang layers is believed to be dependent onparameters such as, but not limited to, the glass composition, thesilver concentration in the glass, the hydrogen partial pressure, thetime and temperature of the ion exchange process, and the time andtemperature of the hydrogen reduction process.

FIG. 5 is the ion profile, potassium or sodium ion concentration in mol%, determined as the oxide, versus depth from the glass surface.Potassium oxide (curve 22) and sodium oxide (curve 24) concentrationswere measured in fusion drawn glass sheets which had undergone hydrogenreduction and the redraw process. For comparison, potassium oxide (curve20) and sodium oxide (curve 26) concentrations in fusion drawn glasssheets which had undergone hydrogen reduction, had been redrawn and weresubsequently treated in an ion-exchange molten metal salt bath whichincluded a potassium nitrate salt. Table III summarizes the curves shownin FIG. 5.

TABLE III Numeral meaning for FIG. 5 Numeral, Species K Ion-exchangeafter redraw 20, K₂O Yes 22, K₂O No 24, Na₂O No 26, Na₂O Yes

The exchange of potassium ions for sodium ions is known to increaseglass strength; however, it was unexpected that an exchange of potassiumions for sodium ions after the formation of Liesegang bands or layerswould be possible because it was believed that the bands or layers wouldblock or interfere with the ion-exchange. As seen from FIG. 5, there isa definite exchange of potassium ions for sodium ions near the surfaceof the glass after treatment in the subsequent ion-exchange molten metalsalt bath. As such, FIG. 5 indicates that the polarizers of the presentdisclosure can be chemically strengthened.

While the disclosure has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the disclosure as disclosed herein.Accordingly, the scope of the disclosure should be limited only by theattached claims.

What is claimed is:
 1. A glass optical polarizer comprising asubstantially bubble free fusion drawn glass comprising two pristineglass surfaces and a plurality of elongated zero valent metallicparticle polarizing layers.
 2. The glass optical polarizer of claim 1,wherein the polarizer has a thickness of about 300 μm or less.
 3. Theglass optical polarizer of claim 1, wherein the polarizer has a contrastratio of at least about 20 dB and a % transmission of at least about 75%over a wavelength range of about 900 nm to about 2,000 nm.
 4. The glassoptical polarizer of claim 1, wherein each of the metallic particlepolarizing layers has a thickness of about 0.2 μm to about 0.5 μm, andwherein each of the metallic particle polarizing layers are situatedwithin about 1.0 μm to about 2.0 μm of at least one the pristine glasssurfaces.
 5. The glass optical polarizer of claim 1, wherein thepolarizer has a thickness of about 200 μm to about 300 μm.
 6. The glassoptical polarizer of claim 1, wherein the polarizer has a contrast ratioof at least about 25 dB.
 7. The glass optical polarizer of claim 1,wherein the elongated zero valent metallic particles comprise a noblemetal.
 8. The glass optical polarizer of claim 7, wherein the noblemetal is selected from the group consisting of copper, gold, iridium,mercury, osmium, palladium, platinum, rhenium, rhodium, ruthenium, andsilver.
 9. The glass optical polarizer of claim 1, wherein the fusiondrawn glass is selected from the group consisting of alkali-containingaluminosilicate glass and alkali-containing aluminoborosilicate glass.10. The glass optical polarizer of claim 1, wherein the fusion drawnglass comprises about 60 mol % to about 70 mol % SiO₂, about 13 mol % toabout 15 mol % Na₂O, about 7.0 mol % to about 15 mol % Al₂O₃, less thanabout 8.0 mol % B₂O₃, less than about 8.0 mol % MgO, less than about 2.0mol % K₂O, less than about 1.0 mol % CaO and less than about 0.5 mol %SnO₂.